1. Field of the Invention
This invention relates to a device for potentiometrically determining the activity (concentrations) of carbonate and hydrogen ions in aqueous liquid samples and various aqueous solutions, especially in body fluids such as blood, blood plasma, blood serum and urine. In particular, this invention relates to a device on which a single drop of each of a reference liquid and a liquid sample are spotted and with which the concentration of carbonates (or carbonate ions) in and the pH value of a liquid sample can simultaneously be determined.
2. Description of the Prior Art
In body fluids, various mechanisms act against exogenous or endogenous acids and bases so that the pH value thereof is kept within the range of 7.40.+-.0.05. The most important chemical buffer action inherent in organisms is the hydrogencarbonate system.
As indicated by the Henderson-Hasselbalch expression: pH=pK+log[HCO.sub.3.sup.- ]-log[.alpha..P.sub.co2 ] The pH value is determined from the concentration of hydrogencarbonate ions (HCO.sub.3.sup.-) and the partial pressure of carbonates (P.sub.CO2).
In the above expression, pK and .alpha. denote the dissociation constant of hydrogencarbonate ions and the solubility coefficient of carbonates, respectively. They are values which ar determined by temperature and pH. The pH, TCO.sub.2 (total carbonate concentration), P.sub.co2 and HCO.sub.3.sup.- concentration can be calculated on the basis of the above expression when at least two of the parameters just mentioned are known.
Abnormalities in the pH are grouped into acidosis in which the pH shifts to the acid side and alkalosis in which the pH shifts to the basic side. According to the causes thereof, they are further divided into metabolic acidosis and alkalosis which are mainly caused by abnormalities in the concentration of HCO.sub.3.sup.- and respiratory acidosis and alkalosis which are mainly caused by abnormalities in the P.sub.co2. When an abnormality in the acid-base equilibrium occurs, the P.sub.co2 and the concentration of HCO.sub.3.sup.- can be calculated on the basis of the above expression if the concentration of carbonates and the pH value are known, and thereby the cause for the abnormality in the pH can be presumed.
Measurements of the concentration of carbonates and the pH value in blood samples should be conducted as soon as possible after the blood samples are collected. This is because the P.sub.co2 in body fluids is more than 100 times as high as that in the air. That is, if the blood samples are not kept from coming into contact with air, the P.sub.co2 thereof will decrease immediately after they have been collected. Thus, the pH value of the blood samples increases according to the Henderson-Hasselbalch expression. Also, even when the blood samples are kept from coming into contact with air, their pH value decreases (changes toward the acidic region) due to an increase in the P.sub.co2 as time passes. As seen from the Henderson-Hasselbalch expression, the concentration of carbonates will yield an error if the pH value fluctuates. However, in most of the conventional measuring devices, the concentration of carbonate ions cannot be measured immediately after the blood is collected, since whole blood cannot be subjected to measurements until after the blood cells are separated therefrom by means of a centrifuge or the like. Also, in order to further examine the acid-base equilibrium, many items such as the pH, the concentration of carbonates, the P.sub.co2 and the concentration of HCO.sub.3.sup.- must be measured.